A color picture tube such as a picture tube used in a color television set substantially comprises an electron gun and a fluorescent surface which converts electron beams into an image. The inside of the picture tube is covered with magnetic shield materials for preventing electron beams from being deflected by terrestrial magnetism.
As such magnetic shield materials, thin steel sheets on which a black treatment or a nickel plating is provided are used, wherein the thin steel sheets are formed in a desired shape by bending and they are sealed to Braun tubes at a temperature of around 600.degree. C. The steel sheets which are used as the magnetic shield materials are required to meet favorable mechanical characteristics such as a favorable formability including bending and a handling strength capable of preventing the deformation of workpieces at the time of conveying the magnetic shield materials before or after the forming operation as well as at the time of piling the workpieces, in addition to excellent magnetic shield properties such as high permeability, low coercive force and high shield efficiency.
To decrease the coercive force while increasing the permeability, the presence of precipitation of carbon, nitrogen, carbide or nitride in the steels which impedes the movement of a ferromagnetic domain wall must be minimized and the grain growth must be promoted while decreasing the grain boundary. Although decreasing of carbon and nitrogen in the steels while increasing the grain growth of the steels provides an improvement of formability since the strength of the steels is decreased, at the time of conveying the steel sheets or workpieces which are produced by bending, they tend to suffer from irregularities even when a slight impact is applied to them or the workpieces tend to be deformed due to the weight of the workpieces piled. Although the handling strength of the steel sheets can be enhanced by grain refining or addition of a certain amount of carbon and nitrogen into the steels so as to precipitate carbide and nitride in the steels, such a method causes deterioration of magnetic characteristics. In this manner, the steel sheets used as the magnetic shield materials must simultaneously meet the excellent magnetic characteristics and the favorable handling strength which conflict with each other.
Conventionally, there have been proposed soft magnetic silicon steel sheets having an excellent handling strength as the materials which have magnetic shielding characteristics. However, such steel sheets have not been in practical use, since it is difficult to apply a black treatment required by color picture tubes to the soft magnetic silicon steel sheets. At present, as the magnetic shield materials, hyper-low carbon aluminium killed steel sheets provided with a black treatment and nickel plated steel sheets provided with a nickel plating have been practically used. Although these steel sheets have excellent magnetic shield characteristics, they do not have a sufficient handling strength.
Accordingly, it is an object of the present invention to provide magnetic shield materials having excellent inner magnetic shield characteristics and an excellent handling strength, and a method for producing such materials and color picture tubes incorporating such magnetic shield materials.